1. Field of the Invention
The present invention relates generally to the technology of measuring the non-methane hydrocarbon concentration in the emissions of an automotive internal combustion engine, and more particularly to the use of catalytic differential calorimetric sensors to monitor the non-methane hydrocarbon oxidation efficiency of an exhaust system's catalytic converter.
2. Description of the Related Art
Catalytic converters have been used on gasoline-fueled automobiles produced in the United States since the mid-1970's for the purpose of promoting the oxidation of unburned hydrocarbons (HCs) and of carbon monoxide (CO). Soon after their introduction, the converters were adapted to promote the chemical reduction of oxides of nitrogen (NO.sub.x). At the present time these converters typically employ small amounts of platinum, palladium and rhodium dispersed over a high surface area particulate carrier vehicle which, in turn, is distributed as a thin, porous coating (sometimes called a washcoat) on the wall of a ceramic monolith substrate. The substrate is typically formed by an extrusion process providing hundreds of thin wall, longitudinal parallel open cells per square inch of cross section. These flow-through catalytic devices are housed in a suitable stainless steel container and placed in the exhaust stream under the vehicle downstream from the engine's exhaust manifold
Under warm, steady-state engine conditions, this conventional catalytic converter containing the precious metal based three-way catalyst (TWC), so called because it simultaneously affects the oxidation of CO and unburned HCs and the reduction of NO.sub.x, effectively and efficiently removes most of the automotive emissions. However, the catalyst system may become malfunctioning after experiencing thermal aging at an unusually high temperature, high exposure to poisoning gases like SO.sub.2, and Pb, etc. Furthermore, new emissions regulations require an extended durability of the catalytic converter from 50,000 miles to 100,000 miles Lastly, as a means to ensure that vehicles meet the certified emission standards throughout the vehicle's operation life, On-Board Diagnostics-II (OBD-II) regulation, as passed by the California Air Resource Board (CARB), calls for continuous monitoring of the efficiency of catalytic converters by direct measurement of the hydrocarbon emission in the exhaust system after the catalyst light-off. Specifically, the monitoring system should be able to indicate when the catalyst system is malfunctioning and its conversion capability has decreased to the point where either of the following occurs: (1) HC emissions exceed the applicable emission threshold of 1.5 times the applicable Federal Test Procedure (FTP) HC standard for the vehicle, and (2) the average FTP Non-methane Hydrocarbon (NMHC) conversion efficiency of the monitored portion of the catalyst system falls below 50 percent.
On the other hand, automotive emissions before the catalyst system has warned up to operational temperatures, namely, cold start emissions comprise the majority of pollution from automobiles. Approaches such as, catalytic converters, close coupled to the engine, which heat and begin to function within a few seconds, electrically heated catalytic converters and in-line adsorbers which temporarily store unburned hydrocarbons until the catalytic converter lights off, have all been proven to be effective solutions for the reduction of cold start emissions. Again, OBD-II regulations require that systems be installed in the exhaust system to directly monitor the functional status of any of these "cold-start" devices during the lifetime of the car (100,000 miles).
The use of hydrocarbon sensors as on-board catalytic efficiency monitors is a relatively new technological area which has generated increasing interest for the auto industry as a result of OBD-II legislation. Generally, the use of a catalytic calorimetric sensor, which measures the effect of the exotherm of the catalyzed oxidation of the oxidizable species including hydrocarbons over supported precious metal catalysts on the resistance of a coil conductor is known.
U.S. Pat. No. 5,408,215 (Hamburg et al.) generally discloses a system comprising: (1) a test chamber remote from the engine exhaust gas stream, (2) means for alternately supplying the chamber with an upstream and a downstream exhaust gas stream sample; (3) a hydrocarbon sensor exposed to the exhaust gas samples in the chamber to produce a signal responsive to the concentration of hydrocarbon in the chamber, and, (4) a means for comparing the downstream and upstream signals to produce a sensed signal for comparison with a reference signal to determine if the converter is faulty.
U.S. Pat. No. 5,265,417 (Visser et al.) discloses a method comprising the steps of: (1) determining the hydrocarbon concentration of the exhaust gas upstream and downstream of the converter by alternately sampling the upstream and downstream exhaust gas and passing the samples to a catalytic differential calorimetric sensor; (2) comparing the hydrocarbon content of the upstream and downstream exhaust gas samples and thereby determining the hydrocarbon conversion efficiency of the catalytic converter.
A shortcoming common to both Visser and Hamburg is the necessary measurement of both an upstream and downstream exhaust gas sample at a position remote from the exhaust gas stream. This is accomplished through the use of a combination of a remote sensing chamber and a valving and delivering system which is capable of delivering alternate samples of the upstream and downstream exhaust As a result of the complexity of these systems they exhibit an increased possibility of system failure during the lifetime of the vehicle; e.g., dirty and/or rusty valving may decrease the accuracy of the HC measurement.
Lastly, U.S. Pat. No. 5,444,974 (Beck et al.) discloses a method diagnosing the performance of the catalytic converter for the oxidation of CO and HC involving producing an electrical signal from a calorimetric sensor located in the exhaust stream downstream of the catalytic converter. The calorimetric sensor is comprised of a first portion bearing an oxidized catalyst for CO, H.sub.2 and HC and an adjacent second portion that is oxidation catalyst-free.
Common to all three of these systems is the disclosure of a non-selective or "total" sensor which not only measures the HC species but also the CO and H.sub.2 species present in the exhaust gas. All three references teach the use of a calorimetric sensor, while Hamburg and Visser additionally disclose the use of a semiconductor-type with a material that adsorbs gases; none of the references provide any teaching as to how to make the sensors selective for HCs alone. Given the fact that a properly functioning catalytic converter, after light-off, typically produces an exhaust gas hydrocarbon concentration, which is typically on the order of tens (or below) ppm, while the CO concentration is typically an order of a magnitude greater, none of these diagnostic systems are capable of directly and selectively measuring HC concentration in this concentration range. Specifically, these sensor systems do not compensate or account for these interfering gases, especially the CO, which are present in concentrations far greater than the HC species. As such, the systems exhibit a reduced ability to accurately measure the HC concentration.